1. Field of the invention
The present invention relates to an optical unit for use in a light beam scanning apparatus. More particularly, the present invention relates to an optical unit in which optical elements serving as components of the optical system of a light beam scanning apparatus that scans an object two-dimensionally are adapted to be detachably mounted in a housing in such a way that the hermeticity of its interior is maintained. Examples of two-dimensional scanning apparatus with a light beam include reading a radiation-produced image from a stimulable phosphor sheet by irradiation with exciting light, and recording an image on a photographic material by irradiation with a light beam.
2. Disclosure of the Prior Art
The most familiar method for obtaining a radiation-produced image as a visible image is to use a radiographic system which is the combination of sensitizing paper and a radiographic film having emulsion layers comprising a silver halide light-sensitive material. Recently, a radiation-produced image converting system using a stimulable phosphor has drawn researchers' attention as a substitute for the radiographic system. Certain phosphors, when exposed to various forms of radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, electron beams and ultraviolet rays, store part of the energy possessed by the radiation. When such phosphors are irradiated with exciting light such as visible light, they produce stimulated emission in a quantity corresponding to the stored energy. Phosphors exhibiting such properties are collectively referred to as stimulable phosphors.
A system has been proposed for recording and reproducing radiation-produced image related information by using a stimulable phosphor. According to this system, the radiation-produced image related information of an object such as the human body is first stored in a sheet having a layer of stimulable phosphor (which is hereinafter referred to as "a stimulable phosphor sheet" or simply as "a phosphor sheet"). The sheet then is scanned with exciting light to produce stimulated emission, which is detected with a photoelectric converter to obtain electrical image signals to be subsequently processed to produce an image of the object which is highly adapted for diagnostic purposes. For this system, reference is made to Unexamined Published Japanese Patent Application Nos. 12429/1980, 11395/1981, 163472/1980, 104645/1981, 116340/1980, etc.
In one application of this system, the radiation-produced image related information stored in the stimulable phosphor and which has been converted to electrical signals is subjected to appropriate signal processing. The resulting electrical signals are used to output a visible image on a recording material such as a photographic material or on a display such as CRT, thereby producing an image of the object which is highly adapted for diagnostic purposes, or which has improved adaptability for image reading.
In order to read image related information from the stimulable phosphor sheet in the radiation-produced image recording and reproducing system described above, or for the purpose of recording said image related information on a photographic material, the stimulable phosphor sheet or photographic material is scanned in a two-dimensional way with a light beam scanning apparatus. While this light beam scanning apparatus performs sub-scanning by transporting the object to be scanned, it achieves main scanning in a direction substantially perpendicular to the direction of transport by deflecting the light from a light source such as a laser oscillator with a light deflector such as a galvanometer mirror. In applying this light beam scanning apparatus to an image reading (reproducing) apparatus, the stimulable phosphor sheet is scanned by excitation with a laser beam and the resulting stimulated emission from the sheet is detected to read the image related information stored in the recording medium, namely, the stimulable phosphor sheet.
In application of the same light beam scanning apparatus to an image recording apparatus, a light beam is modulated typically with an accousti-optical modulator (AOM) on the basis of an image related information signal and the intended image related information is recorded by scanning a recording medium (e.g. a photographic material) with the modulated signal.
Whether image related information is reproduced or recorded with the above-described light beam scanning apparatus, extremely high levels of precision are required not only with respect to scanning parameters (e.g., scanning speed and the amount to be scanned) but also in terms of the quantity and luminance of the light beam with which the object to be scanned in sheet form is irradiated.
A laser, light deflector, optical filter, reflector mirror and other optical elements that make up the optical system of a light beam scanning apparatus are highly sensitive to dirt or dust particles or other forms of soil. If such foreign matter is deposited on the optical elements, the light beam is subject to random reflection or produces a shadow that causes unevenness in the quantity and luminance of the light beam. If the object to be scanned in sheet form is irradiated with alight beam whose quantity or luminance is erroneous, abnormal or uneven, the reproduced or recorded image related information can either lack sharpness due to unevenness or turn out to be erroneous. In other words, the finally reproduced image might produce clear spots or shadows that prevent the viewer from making the correct diagnosis.
The light beam scanning apparatus is usually enclosed with the housing of a radiation-produced image reading or recording apparatus and the chance of dirt or dust ingress is fairly small. However, when the housing is opened to change optical elements, not only does the ingress of dust or dirt occur, but also the dirt particles that have built up in various parts of the housing as a result of prolonged use of the apparatus will become suspended in the atmosphere, and the deposition of dirt or dust particles or other forms of soil on various optical elements of the light beam scanning apparatus is unavoidable.